Magnetic resonance measurements involve observing the interaction of magnetic moments of atomic nuclei (e.g., nuclear spins) with an external magnetic field.
When excited by an external alternating electromagnetic field around the axis of magnetic field orientation, nuclear spins align themselves in the external agnetic field and precess at a Larmor frequency that depends on the value of the magnetic moment of the nucleus and the external magnetic field. The atomic nuclei then generate an electromagnetic alternating field at the Larmor frequency.
The external alternating electromagnetic field used to excite the nuclear spins is projected into a sample or into a patient via one or a plurality of antenna arrays. One antenna array is a body coil that encircles the patient or the sample. However, local coils that are disposed directly on the patient or on the sample may be used. The electromagnetic field generated by the atomic nuclei is likewise received by the antenna arrays. The same antenna array may receive the signal that has been generated. Alternatively, the nuclear spins may be generated with one type of antenna and the electromagnetic alternating field generated by the atomic nuclei may be received using a different type of antenna.
The efficiency of projection and the sensitivity of reception are dependent on a plurality of characteristics of the antenna array (e.g., the electrical characteristics thereof). Characteristics of the antenna array may be the resonant frequency or the impedance. These characteristics of the antenna array are also dependent on the temperature of the antenna array and the components thereof. Thus, the inductance of a coil or the capacitance of a capacitor may be changed as a result of thermal expansion.
In the construction of the antenna arrays, the elements used may have a temperature coefficient that is equal to or close to zero for the mechanical or electrical characteristics.
The consequences of changes are also offset by control mechanisms. For example, lower reception sensitivity and/or transmission efficiency when the resonant frequency has changed may be compensated for by greater transmitting speed or input amplification.
However, the electrical characteristics may not always be kept constant using components having a low temperature coefficient because the characteristics of the antenna array also depend on the environment.
A body coil may be provided on a cylindrical element that is disposed concentrically between the patient or the sample and the gradient coils. The gradient coils do not prevent alternating electromagnetic fields from being beamed down onto the patient. In order to reduce external interactions with the gradient coils (e.g., to prevent irradiation and absorption of high-frequency energy in the gradient coils), a high-frequency shield may be disposed on the inside of a supporting base for the gradient coils. The shield extends between the gradient coils and the body coil. The body coil and the shield interact. For example, facing metal surfaces of the body coil and the shield effect a capacitive coupling. The electromagnetic waves that are transmitted by the antenna array generate eddy currents in the shield. Since the distance between the body coil and the shield changes if the gradient coil, together with the supporting base and the shield located thereon, becomes hotter, the electrical characteristics of the body coil change. The electrical characteristics change even if the body coil were to have a constant temperature or were configured with a temperature coefficient equal to zero.